1. Field of the Invention
The invention relates to explosive detectors. In particular the invention relates to a nuclear technique for monitoring objects such as luggage and parcel items and to screen such items for the presence of explosives materials.
The invention concerns a non-invasive method of inspection which involves subjecting the items to a thermal neutron environment and observing the gamma capture reaction.
It is an object of the present invention to provide a method and apparatus for non-invasively inspecting luggage by detecting gamma radiation emitted by selected elements in response to neutron irradiation.
The invention also has for an objective to positively detect the presence of explosives material by identifying certain characteristic elements. Further objectives include to pre-empt countermeasures which might be taken to conceal the explosives material, e.g. shielding; and to minimise false alarms while ensuring that quantities of material above a predetermined minimum do not go undetected.
Still further objectives of the system design include minimum system weight and minimum occupied floor area for the inspection system, source shielding and the baggage handling system. The baggage handling system is also intended to be capable of reaching a throughput of at least 10 bags/minute per channel in a multi-channel facility.
2. Description of the Prior Art
There are already in existence several types of non-invasive baggage inspection systems. The most familiar to air travellers is the ubiquitous x-ray apparatus. This type relies entirely on the visual skills and vigilance of human operators to spot suspicious objects. The systems have no inherent capability to detect explosives material itself.
There are a number of other systems designed to sense explosives material but each has perceived drawbacks which the present arrangements seek to either avoid altogether or to improve upon. The systems considered most effective use neutrons as the inspection medium. Of these there are three types of source each of which gives rise to a characteristic problem specifically addressed by the present invention.
In a first of these systems neutrons are produced by a deuterium-tritium reaction which takes place inside a containment vessel, generally referred to as a "reaction tube". These "tubes" are expensive to replace. Although the source is controllable the major drawback stems from the energy of the reaction which produces neutrons in a narrow energy band at 14 MeV only. These neutrons are too fast for the purpose and have to be moderated. At such energies shielding has to be very bulky.
A second type of system uses a continuous source, for example a californium isotope Cf.sup.252. This produces neutrons having a useful spread of energies in the range 0.5-14 MeV with a mean about 2 MeV. Unfortunately it is a continuously radio-active source. This is undesirable from a security standpoint. The source is effectively uncontrolled, and because of the high energy of the neutrons requires bulky shielding.
The third type of source, also controllable, comprises a radio frequency quadrupole accelerator, which will be subsequently referred to as an RFQ accelerator. This impinges a beam of deuterons onto a beryllium target. It generates a reasonably monoenergetic neutron beam at about 7 MeV. The neutrons are still more energetic than required so a considerable degree of shielding and moderation is still necessary.
The advantages of the present invention which will be apparent from the subsequent description. The neutrons produced are low energy reducing shielding requirements. Other advantages includes easy source control, and the neutron source has only low and soft residual radioactivity. Also a high neutron flux at low proton current allows multi-channel inspection to raise total system throughput.